As with most communication systems, a receiver within a CDMA communications network is more challenging to design and test than the corresponding transmitter. In the following text the term CDMA refers to both the CDMA family of standards (e.g., IS-95, IS-2000, etc.) as well as the code division multiple access technology in general. Transmit functionality is relatively straightforward to implement based on standard CDMA specifications. Although performance targets are specified for the receiver, the actual design configuration is left to the communication system's provider to implement. A difficulty arises with the reverse link where the reverse data is received from a mobile via a radio frequency (RF) link and it is not possible to control the baseband data sent to the modem. This uncontrollability leads to unreliable debugging capabilities of the reverse link. Most of the existing debugging methodology for the reverse link either involves using some sort of access probe (i.e., a mobile transmission attempt) in the form of either a real mobile or a mobile simulator. While adequate for basic testing, these solutions have been not accurate enough for debugging any issues with the reverse link. Hence, an accurate diagnostic tool is needed to provide a deterministic signal that can be used for accurate debugging capability.
In field operation, there exists a “sleepy” base station problem in situations where a given sector on the base station may stop detecting valid access probes from mobiles. Here, the preamble portion is detected and either some or all of the message capsule is received by the modem. However, the reassembled Layer 2 encapsulated PDU fails the CRC check that is embedded in the frame. Consequently, the system discards the received frame data. Because all the access probes are like this when the system is in a bad state, no new calls can be handled. The system loading will decrease to zero as pre-existing calls are dropped. The system lies in a dormant state until the sector is re-initialized on the modem. The cause of this problem remains not well understood. Although the occurrence of the problem is rare, the field impact is quite serious. There may be no service for days or weeks in a given sector as long as the condition remains undetected by the wireless operator. Given the severity of having the system dwell in a bad state for prolonged periods of time before it is corrected via manual intervention, it is imperative to provide a detection mechanism for the sleepy base station issue.
Oftentimes, communications systems cannot detect mobiles (i.e., access probes) in close relation to the base station. Here, if the search window is not set correctly on the base station modem then the system might not be able to detect mobiles that are close to the base station RF antenna. In such instance, there is a hole in the coverage region centered around the RF antenna. This can happen if the calibration tables in the modules are incorrect or if the wireless operator has entered a wrong delay value for the system. This will result in a wrong calculation for forward distribution delay/reverse distribution delay (FDD/RDD) causing the base station modem to start correlating the received signal at a point that is too late in time to see the mobile signal. Effectively, mobiles close to base station have less than zero path offset relative to forward link timing which leads them to be undetected.
Still further, verifying CDMA system operation has been ineffective using either an actual mobile or existing mobile simulators as access probes. The timing of the CDMA forward link is tied to a global positioning system (GPS) timing reference signal (called Even-sec. signal or 2-sec. signal in CDMA terms). Hence, the base station transmits signals that have a known fixed relationship with regard to absolute time. A vector spectrum analyzer (VSA) can be used to verify the integrity of the forward link physical layer of a base station. Many problems can be diagnosed using a VSA without the need for a mobile terminal (e.g., incorrect FDD setting, poor RF signal quality, baseband data corruption, etc.)
The timing of the CDMA reverse link, however, is derived from the forward link signal that the mobile terminal receives. The physical distance and time varying channel conditions between the transmitter and receiver adds both propagation delay and timing uncertainty to the reverse link. This can create problems when trying to determine whether or not the search window is set correctly in the receive modem. In general, debugging problems with the reverse link physical layer is difficult given the number of things that can go wrong (e.g., incorrect RDD setting, no RF signal going to the radio, wrong system configuration, etc.). This difficulty is inherent in CDMA signal processing because the noise-like property of these signals makes their detection, and hence the debugging of reverse link issues, a challenge. It is nontrivial to send a test signal over the reverse link in an open loop manner and have the base station (i.e., modem) detect such test signal. Proper timing synchronization and system parameter configuration are required at the signal source. Consequently, a specially devised mobile terminal is often used for reverse link testing. While this may suffice for lab test purposes, it is not feasible to deploy a test mobile with every base station for an in field test application as this adds significantly to the cost of the final product.
Yet still further, calibration of finger delay data in communications systems can be problematic. Accurate finger delay data is useful for verifying modem operation and for determining the position of the mobile terminal for location-based services. Because the timing of the access probe generator is tied to the even second reference signal (2-sec. or EVEN_SEC), such timing can be used to accurately calibrate finger delay data reported by the modem. For example, there is a slight difference in the finger delay values reported by various Channel Element Modules (CEMs). Also, there are delay variations in the system that depend on things such as HSSL timing and hardware initialization conditions (e.g., receive channelizer). These can introduce some variation in the hardware delays that may or may not be accounted for in the RDD.
It is important that any problem condition within the communications system be detected quickly. Although it might be possible for software to try to infer a problem condition by monitoring call logs, a detection method based on call statistics requires that sufficient data be gathered over a large time interval in order to make an accurate assessment. This implies that the system might dwell in a bad state for a relatively long time before a loss of service condition is recognized. Also, it is not possible to distinguish with certainty between a situation in which a system is not getting any call attempts because there are in fact no mobiles attempting to do so and that of a system being in a bad state. This ambiguity is inherent to CDMA due to the noise-like property of the received signal.
It is, therefore, desirable to provide a robust and cost-effective testing mechanism for setup and monitoring of CDMA base stations.